In the oil and gas extraction industry, hydrocarbons are extracted from a subterranean formation through bores which are drilled from surface level to intersect the formation. In many circumstances, a number of vertical bores are required for efficient and effective extraction of hydrocarbons from a single formation, which often necessitates a corresponding number of surface drilling locations, which can be undesirable, particularly in offshore drilling operations. Methods, however, exist which allow non-vertical bores to be drilled permitting a wide area to be accessed from a single surface drilling location. Such methods are commonly referred to as directional or controlled trajectory drilling.
During directional drilling operations, a curved or deviated bore may be drilled by placing a slight bend in the drilling assembly, referred to as a bent sub assembly, and orientating the bend in the required direction. For example, if the bend points upwards, the well bore will gain inclination angle. Likewise, if the bend points downwards, the well will drop angle and tend to return to a vertical plane. If the bend is used to point the drill bit left or right, the well bore will change direction accordingly to the left or right.
Rotation of the drill bit is normally achieved by rotating the drill string from surface level. When a bent sub assembly is present, the rotation of the whole drill string negates the effect of the bent sub assembly.
However, it is conventional to drive the drill bit during directional drilling using a downhole positive displacement mud motor which normally comprises a long section of internally “twisted” pipe with a similarly twisted rotor positioned therein. The flow of drilling fluid through the twisted section will turn the rotor which is connected to the drill bit by a flexible steel rod which passes through the bent sub assembly. In this way, the drill bit is turned without the bent sub assembly rotating and without the requirement for the whole drill pipe to be rotated from the surface.
However, when the bit makes contact with the rock face at the bottom of the bore, the torque generated by the mud motor has an equal and opposite reactive torque which will cause the drill string to twist or rotate back to surface level. The twist is normally significant and makes control of the angle at which the bent sub points the drill bit difficult to set and maintain.
Furthermore, when the drill string is not rotated, a situation known as “stick slip” occurs which can potentially damage the tooling. Stick slip occurs because the weight applied to a stationary drill string to advance a drill bit has to overcome static friction between the drill string and the bore wall; the non-rotating string tends to stick in the bore, such that weight or force has to be applied to move the string forward. The string will then often “unstick” suddenly, and slip forward, forcing the drill bit into the end of the bore and often stalling the mud motor. It is therefore preferred that the drill string also be rotated during all drilling operations such that a lower, dynamic friction has to be overcome, allowing for smoother drilling.
It is known to provide directional drilling while rotating the entire drill assembly (“rotary directional drilling”), however, this requires additional and sometimes complicated and expensive downhole assemblies to maintain the bent sub in its desired orientation.
During directional drilling, it is essential that the direction in which the bent sub is pointing is known at all times, to ensure that the bore is being drilled in the correct direction and that adjustment to the orientation of the bent sub may be made as soon as an error is detected. In previously proposed rotary directional drilling systems, such monitoring is commonly achieved using dedicated complex electrical systems which are provided in addition to the Measurement-While-Drilling (MWD) systems already provided within the bottom hole drilling assembly (BHA), thus increasing the complexity and expense of monitoring equipment which must be provided.
It is among the objectives of the embodiments of the present invention to provide directional drilling apparatus which obviates, or at least mitigates the aforementioned problems with the prior art.